Continuously variable hydrostatic power transmissions have been available for many years and are in use in applications in which their noise and inefficiency have not been seriously objectionable, such as lawn and garden tractors. However, such noise and inefficiency have recently become unacceptable in lawn and garden equipment, and there are numerous other applications for economical low and medium power continuously variable transmissions that require efficient and quiet operation.
This invention provides an economical continuously variable power transmission that is efficient and quiet.
The continuously variable power transmission according to this invention includes a fixed support plinth at one end by which the transmission is mounted to a fixed support structure, and an output shaft at the other end. A pintle having a flat radial slab and an axial arbor is bolted to the pinth and supports a radial piston pump rotor on one end of the arbor and a radial piston motor rotor on the other end of the arbor. Cylindrical cam rings are mounted around the pump and motor rotors to engage piston balls in the pump and motor rotor cylinders for pumping and torque generation. A driven pulley, having a drive surface engaged with a driving element for coupling rotating mechanical power to the transmission, surrounds the transmission and constitutes its exterior shell within which working fluid such as oil is contained. The driven pulley is supported for rotation on bearings at its two ends. At the support end, the pulley is supported on the plinth, and at the other end is supported on a fixed housing attached to the plinth. The output end of the driven pulley is coupled to a ring gear of a planetary gear set having a planet carrier with planet gears engaged between the ring gear and a sun gear. The sun gear is coupled to and drives the radial piston pump rotor, and the planet carrier is coupled to the output shaft, so the reaction torque from the torque applied to the pump rotor is applied directly to the output shaft. Working fluid pressurized by rotation of the pump is conveyed through channels in the pintle arbor to the motor rotor to pressurize the motor cylinders and generated torque in the motor which is carried via a motor drum back to the planet carrier and thence to the output shaft. The transmission ratio is set by adjusting the radial position of the cam rings which are mounted on a pivot pin at the lower end of the pintle slab and are tilted to the desired radial positions by action of a control system having actuator pistons mounted in upper part of the pintle slab. The cam rings may be coupled together at the top, opposite the pivot pin, by a lever rod mounted for swiveling in the top of the pintle slab, so tilting of the motor cam ring in one direction causes the lever rod to swivel about its swivel mounting in the pintle and tilt the pump cam ring in the opposite direction.